Osnr montoring method and apparatus for the optical networks

ABSTRACT

The invention relates to method and apparatus that automatically monitors each channel&#39;s optical signal-to-noise ratio (OSNR) using optical filter and polarization extinction method in wavelength division multiplexing (WDM) scheme-based optical transmission systems.  
     In the present invention, OSNR is simply measured using optical filter by comparing amplified spontaneous emission (ASE) over the signal band, of which bandwidth has changed, while leaving signal intensity intact, with that original signal. Also, OSNR measurement is allowable over a wider range of OSNR by minimizing the ratio of signal to ASE over the signal band using polarization extinction method.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to OSNR monitoring method andapparatus for the optical networks. More particularly, the inventionrelates to method and apparatus that automatically monitors eachchannel's optical signal-to-noise ratio using optical filter andpolarization extinction method in wavelength division multiplexingscheme-based optical transmission systems.

BACKGROUND ART

[0002] Recently, as Wavelength Division Multiplexing (WDM) opticaltransmission techniques come into practical use, the transmissioncapacity of optical transmission systems has rapidly enhanced up to over1 Tb/s.

[0003] In order to operate and maintain reliably the opticaltransmission systems of such an ultra-large capacity, it is veryimportant to accurately monitor Optical Signal-to-Noise Ratio (OSNR) oftransmitted signal.

[0004] OSNR is defined as the ratio of signal power to noise powercontained over the signal band. So transmission performance of anoptical transmission system can be represented by OSNR.

[0005] In WDM optical communication networks, since each channel comesthrough different paths, it has different OSNR each other.

[0006] Therefore, it is not accurate to linearly predict noise intensityover the signal band from the intensity of amplified spontaneousemission (ASE) outside the signal band as proposed in the past. [Referto H. Suzuki and N. Takachio, “Optical signal quality monitor built intoWDM linear repeaters using semiconductor arrayed waveguide gratingfilter monolithically integrated with eight photodiodes,” ElectronicsLetter, Vol. 35, pp. 836-837, 1999.]

[0007] Recently, there is proposed a method where each channel's OSNR ismeasured using polarization characteristics of optical signal and ASE.[Refer to D. K. Jung, C. H. Kim, and Y. C. Chung, “OSNR monitoringtechnique using polarization-nulling method,” OFC 2000, Baltimore, March2000, Paper WK4.]

[0008] However, the above method is limited in its application scope dueto the phenomenal fact that signal's degree of polarization (DOP) islowered after transmission because of polarization mode dispersion (PMD)and nonlinear birefringence. [Refer to J. H. Lee, D. K. Jung, C. H. Kim,and Y. C. Chung, “OSNR monitoring technique using polarization-nullingmethod,” IEEE Photon. Technol. Lett., vol. 12, no. 1, pp 88-90, 2001.]

DISCLOSURE OF INVENTION

[0009] It is an object of the present invention to resolve theaforementioned problems and, therefore, to provide a simple OSNRmonitoring method and apparatus for the optical networks using anoptical filter of which passband is identical to that of signal, bycomparing ASE over the signal band, of which bandwidth has changed,while leaving signal intensity intact, with that original signal.

[0010] It is another object of the present invention to provide OSNRmonitoring method and apparatus for the optical networks which allowmonitoring over a wider range of OSNR by minimizing the ratio of signalto ASE over the signal band using polarization extinction method.

[0011] It is also an object of the present invention to provide aneconomic and efficient OSNR monitoring method and apparatus for theoptical networks which allow automatic monitoring using optical filter,automatic polarization controller, and polarization beam splitter.

[0012] It is further another object of the present invention to providean economic and efficient OSNR monitoring method and apparatus for theoptical networks which exclude the effect of polarization modedispersion and nonlinear birefringence that limit the usage scope ofmonitoring method using only polarization extinction method in the priorart.

[0013] To achieve the aforementioned object, the present inventionprovides OSNR monitoring method for the optical networks that ischaracterized to measure OSNR by changing ASE bandwidth using an opticalfilter of which passband is identical to the wavelength of each WDMsignal transmitted in the WDM optical transmission system.

[0014] To achieve the aforementioned object, the present inventionprovides OSNR monitoring apparatus for the optical networks whichcomprises; 1:1 directional coupler splitting ASE-contained opticalsignal in WDM optical communication system into two; optical filterchanging ASE bandwidth for one of the above two signals; photodetectorconverting, into voltage signals, the other optical signal and theoptical signal which has changed the bandwidth; and computer measuringOSNR using voltage of the photodetector as input.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 is a diagram representing the principle of monitoringmethod for optical signal-to-noise ratio (OSNR) according to anembodiment of the present invention.

[0016]FIG. 2 is a block diagram of OSNR monitoring apparatus implementedaccording to an embodiment of the present invention of OSNR monitoringmethod.

[0017]FIG. 3 is a block diagram of experimental system to prove thevalidity of OSNR monitoring method according to an embodiment of thepresent method.

[0018]FIG. 4 is a graph showing errors of OSNR measured by OSNRmonitoring apparatus according to an embodiment of the present inventionagainst OSNR measured by optical spectrum analyzer.

[0019]FIG. 5 is a diagram representing the principle of method toenhance the accuracy of OSNR monitoring method according to the presentinvention using polarization extinction method.

[0020]FIG. 6 is a block diagram of experimental system to prove thevalidity of OSNR monitoring method according to the present method usingpolarization extinction method.

[0021]FIG. 7 is a graph showing errors of OSNR measured using OSNRmonitoring apparatus according to the present invention usingpolarization extinction method against OSNR measured by optical spectrumanalyzer.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022]FIG. 1 is a diagram representing the principle of OSNR monitoringmethod according to the present invention.

[0023] If optical signal containing amplified spontaneous emission (ASE)is split into two by using directional coupler (10) and then one of thetwo is passed through optical filter (20) whose center wavelength isidentical to the signal wavelength, the signal intensity stays the samebut the ASE bandwidth gets changed.

[0024] Therefore, measured optical power (P₁, P₂) can be described bythe following equations.

[0025] [Eqn.1]

P ₁ =P _(signal) +S _(ASE) B ₁

[0026] [Eqn.2]

P ₂ =P _(signal) +S _(ASE) B ₂

[0027] Here, P_(signal) represents signal intensity in watt, S_(ASE)represents power density of ASE in watt/nm, and B₁ and B₂ representbandwidth of ASE in nm, respectively.

[0028] Using Eqn.1 and Eqn.2 above, signal intensity and power densityof ASE can be represented by the following equations, respectively.

[0029] [Eqn.3]

P _(signal)=(P ₂ B ₁ −P ₁ B ₂)/(B ₁ −B ₂)

[0030] [Eqn.4]

S _(ASE)=(P ₁ −P ₂)(B ₁ −B ₂)

[0031] Therefore, after obtaining OSNR by using Eqn.3 and Eqn.4, OSNR isfinally represented by the following equation with given resolution ofBr(nm). $\begin{matrix}{{OSNR} = {\frac{P_{signal}}{S_{ASE}B_{r}} = \frac{{P_{2}B_{1}} - {P_{1}B_{2}}}{( {P_{1} - P_{2}} )B_{r}}}} & \lbrack {{Eqn}.\quad 5} \rbrack\end{matrix}$

[0032] The OSNR monitoring method according to the present methodutilizes the fact that if optical signal containing ASE is passedthrough optical filter (20) whose center wavelength of passband isidentical to signal wavelength, then signal intensity stays the same butASE bandwidth gets changed. Therefore, signal intensity and powerdensities of ASE are obtained by comparing powers before and afterpassing through the optical filter (20), and OSNR is determined byinputting these values into Eqn.5.

[0033]FIG. 2 is a block diagram of OSNR monitoring apparatus (A)implemented according to the present invention of OSNR monitoringmethod.

[0034] To split the input optical signal into two, 1:1 directionalcoupler (10) is used.

[0035] One of the split signals is converted into voltage byphotodetector (30 a), amplified by logarithmic amplifier (40 a),converted into digital signal by A/D converter (50), and then input tocomputer (60).

[0036] The other signal, after passing through optical filter (20), issimilarly converted into voltage by photodetector (30 b), amplified bylogarithmic amplifier (40 b), converted into digital signal by A/Dconverter (50), and then input to computer (60).

[0037] Now, the computer (60) is ready to measure OSNR by applying thetwo input voltages to Eqn.5.

[0038]FIG. 3 is a block diagram of experimental system to prove thevalidity of OSNR monitoring method according to the present method.

[0039] Wavelength tunable laser (72) is provided to generate opticalsignal, while erbium-doped fiber amplifier (EDFA) is provided as source(74) for generating ASE.

[0040] Once 1:1 directional coupler (10 a) combines signal and ASE, then1:1 directional coupler (10 b) again splits the combined signal and ASEinto two.

[0041] One of the two is input to optical spectrum analyzer (76), andOSNR is measured by linear prediction method.

[0042] Here, since signal bandwidth is narrow while ASE bandwidth iswide and flat, OSNR measured by linear prediction can be said to beaccurate.

[0043] The other of the two is passed through arrayed waveguide grating(78) for demultiplexing, and then input to OSNR monitoring apparatus (A)according to the present invention for measuring OSNR.

[0044] The channel spacing and passband of arrayed waveguide grating(78) used as demultiplexer are 1.6 nm and 0.944 nm, respectively, andpassband of optical filter (20) used for OSNR monitoring apparatus (A)is 0.668 nm.

[0045] Using optical variable attenuators (75 a,75 b) installeddownstream each of wavelength tunable laser (72) and ASE source (74),OSNR can be varied by increasing or decreasing signal power and ASEquantity.

[0046]FIG. 4 is a graph showing of error results (Error_(av)), averagedover 200 measurements, of OSNR measured using OSNR monitoring apparatus(A) according to the present invention against OSNR measured by opticalspectrum analyzer (76).

[0047] As can be seen in the results, two measured OSNR's are in goodagreement, the maximum error being within 1 dB.

[0048] However, it is observed that variance of errors (Error_(var))tends to increase as OSNR becomes large.

[0049] This is attributed to the fact that, since ASE intensity becomessmaller compared to signal as OSNR becomes large, because of the limitof measurement resolution, i.e., resolution of A/D converter (50),larger errors are involved in OSNR calculation.

[0050] However, this problem can be resolved by averaging measuredOSNR's over many times.

[0051] In the present invention, it is proposed to enhance the accuracyby keeping OSNR low enough using polarization extinction method.

[0052]FIG. 5 explains the principle of high-accuracy OSNR monitoringmethod using both optical filter (20) and polarization extinction methodsimultaneously.

[0053] After passing ASE-contained signal through automatic polarizationcontroller (80), the signal is split into two perpendicularly polarizedelements by polarization beam splitter (82).

[0054] Linear optical transformer may be used in place of the abovepolarization beam splitter (82).

[0055] One of the two split polarized elements is again split usingdirectional coupler (10) into two, one of which is passed trough opticalfilter (20).

[0056] Here, in order to make ASE bandwidth after passing narrower thanthat before passing, bandwidth of optical filter (20) is made to benarrower than that of arrayed waveguide grating used as multiplexer.

[0057] Now, the three split powers can be represented as follows:

[0058] [Eqn.6]

P ₁ =P _(signal)(1−ε)+0.5S _(ASE) B ₁

[0059] [Eqn.7]

P ₂=0.5P _(signal)ε+0.25S _(ASE) B ₁

[0060] [Eqn.8]

P ₃=0.5P _(signal)ε+0.25S _(ASE) B ₂

[0061] Here, ε represents polarized component of signal that is excludedfrom P₁.

[0062] Each of the above three powers (P₁,P₂,P₃) is measured byphotodetector (30 a,30 b,30 c), amplified by logarithmic amplifier (40a,40 b,40 c), converted into digital signal by A/D converter (50), andthen input to computer (60) for use in OSNR calculation.

[0063] On using Eqn.6 through Eqn.8, OSNR is obtained as in Eqn.9.$\begin{matrix}{{OSNR} = {\frac{P_{signal}}{S_{ASE}B_{r}} = {{\frac{( {P_{1} + {2P_{2}}} )}{4( {P_{2} - P_{3}} )}\frac{B_{1} - B_{2}}{B_{r}}} - \frac{B_{1}}{B_{r}}}}} & \lbrack {{Eqn}.\quad 9} \rbrack\end{matrix}$

[0064] Here, for the sake of more accurate OSNR measurement,polarization controller (80) before polarization beam splitter (82) isautomatically adjusted to minimize ε so that signal intensitiescontained in P₂ and P₃ are minimized.

[0065] In other words, if quantity of signal component contained in thetwo measurements (P₂ and P₃) is made smaller than or comparable to ASEintensity, ASE intensity can be predicted more accurately even with thelimited resolution of measuring devices.

[0066] Therefore, if polarization extinction method is used, one canimplement a more accurate OSNR monitoring method.

[0067] Here, if extinction ratio of polarization beam splitter (82) isassumed infinity, ε becomes zero.

[0068] In this case, the system reduces to the same constitution as inthe prior art, and OSNR can be calculated without Eqn.8.

[0069] However, since polarization beam splitter (82) has a finiteextinction ratio and polarization adjustment is not complete in reality,ε does not become zero.

[0070] Also, non-nullified value of ε can be attributed to polarizationmode dispersion and nonlinear birefringence.

[0071] Polarization mode dispersion means time difference between signalcomponents proceeding along two polarization axes caused due topolarization property of fiber or optical elements when optical signalis transmitted over optical lines.

[0072] Sine this polarization mode dispersion is sensitive tosurrounding environment such as ambient temperature and atmosphericpressure, it changes as time goes on.

[0073] Nonlinear birefringence is type of birefringence that arises dueto change of refractive index of fiber caused by optical signalintensity. Therefore, when a multiplicity of intensity modulated opticalsignals are transmitted on single fiber simultaneously, nonlinearbirefringence changes the polarization state of adjacent channelrapidly.

[0074] Since this change of polarization depends on polarizationalrelation between channels, the effect due to nonlinear birefringencealso changes as time goes on.

[0075] Therefore, ε resulted from polarization mode dispersion andnonlinear birefringence changes as time passes, which causesunpredictable errors in OSNR measurement using only polarizationextinction method.

[0076] In case of OSNR monitoring method according to the presentinvention, on the other hand, even though ε varies as time passes,accurate OSNR can be predicted by considering the effect of change of εin real time.

[0077] Therefore, OSNR monitoring method according to the presentinvention can be credited to eliminate the effects of polarization modedispersion and nonlinear birefringence that have been limitation factorsof OSNR monitoring method using polarization extinction method in theprior art.

[0078]FIG. 6 is a block diagram of experimental system to prove thevalidity of OSNR monitoring method according to the present method usingpolarization extinction method.

[0079] Here, WDM optical signal is provided by multiplexing outputsignal from eight distributed feedback lasers (84 a, . . . ,84 n) withchannel spacing of 1.6 nm by use of arrayed waveguide grating (78 a),while ASE is provided by using erbium-doped fiber amplifier as ASEsource (74).

[0080] Signal and ASE coupled by 1:1 directional coupler (10 a) is againsplit by 1:1 directional coupler (10 b) into two.

[0081] One part of the two is input to optical spectrum analyzer (76),measuring OSNR by linear prediction method.

[0082] Here, since signal bandwidth is narrow while ASE bandwidth iswide and flat, linear prediction can be said to yield accuratemeasurement results.

[0083] The other part of the two is passed through arrayed waveguidegrating (78 b) for demultiplexing and input to OSNR monitoring apparatus(A) for measuring OSNR.

[0084] The arrayed waveguide grating (78 b) used as demultiplexer haschannel spacing and passband of 1.6 nm and 0.944 nm, respectively.

[0085] Using optical variable attenuators (75 a,75 b) installeddownstream each of ASE source (74) and arrayed waveguide grating (78 a),OSNR can be varied by increasing or decreasing signal power and ASEquantity.

[0086]FIG. 7 is a graph showing errors of OSNR measured using OSNRmonitoring apparatus (A) according to the present invention usingpolarization extinction method against OSNR measured by optical spectrumanalyzer (76).

[0087] As shown in the results, measured OSNR is in good agreement forall eight channels, the maximum error being as low as 0.3 dB.

Industrial Applicability

[0088] As shown above, since OSNR monitoring method according to thepresent invention is implemented using only optical filter anddirectional coupler, its constitution is simple and economical.Moreover, it can be improved for monitoring over wider range simply byadding automatic polarization controller and polarization beam splitter.

[0089] The OSNR monitoring method according to the present invention isproposed to overcome, by simply adding optical filter, the limitation ofOSNR monitoring method using polarization extinction method thatincludes polarization mode dispersion and nonlinear birefringence.

1. An OSNR monitoring method for the optical networks characterized; tomeasure OSNR by changing ASE bandwidth using optical filter whosepassband is identical to the wavelength of each WDM signal transmittedon WDM scheme-based optical transmission system.
 2. An OSNR monitoringmethod for the optical networks of claim 1, wherein OSNR is continuouslymeasured by repeating a series of processes comprising; process in whichsignal containing said ASE is split into two, process in which one ofthe two is measured as the first power, process in which the other ofthe two is measured as the second power after reducing the ASE bandwidthusing said optical filter whose passband is identical to the opticalsignal wavelength, and process in which signal intensity and ASEintensity are obtained from said two measurements of the first power andthe second power.
 3. An OSNR monitoring method for the optical networksof claim 2, wherein OSNR is measured under the condition thatsignal-to-noise ratio is minimized by using polarization characteristicof said optical signal.
 4. An OSNR monitoring method for the opticalnetworks of claim 2, wherein OSNR is measured by the following Equation10. $\begin{matrix}{{{OSNR} = {\frac{P_{signal}}{S_{ASE}B_{r}} = \frac{{P_{2}B_{1}} - {P_{1}B_{2}}}{( {P_{1} - P_{2}} )B_{r}}}},} & \lbrack {{Eqn}.\quad 10} \rbrack\end{matrix}$

where P₁ and P₂ indicate the first and the second measured powers,P_(signal) indicates signal intensity in watt, S_(ASE) indicates powerdensity of ASE in watt/nm, B₁ and B₂ indicate bandwidths of ASEassociated with the first and the second powers in nm, and B_(r)indicates resolution in nm, respectively.
 5. An OSNR monitoring methodfor the optical networks characterized; to measure OSNR under thecondition that signal-to-noise ratio is minimized by using polarizationcharacteristic of each WDM signal transmitted on WDM scheme-basedoptical transmission systems, and in measuring ASE power containingsignal components due to polarization mode dispersion and nonlinearbirefringence, intensity of said signal component contained in ASE isremoved by using an optical filter whose passband is identical to thesignal wavelength.
 6. An OSNR monitoring method for the optical networksof claim 5, wherein OSNR is continuously measured by repeating a seriesof processes comprising; process in which said ASE is split into twoperpendicularly polarized components, process in which one of the two ismeasured as the first power, process in which the other of the two isagain split into two, one of which is measured as the second power andthe other of which is measured as the third power after reducing the ASEbandwidth using said optical filter whose passband is identical to theoptical signal wavelength, and process in which signal intensity and ASEintensity are obtained from said three measurements of the first,second, and third powers.
 7. An OSNR monitoring method for the opticalnetworks of claim 6, wherein OSNR is measured by the following Equation11. $\begin{matrix}{{{OSNR} = {\frac{P_{signal}}{S_{ASE}B_{r}} = {{\frac{( {P_{1} + {2P_{2}}} )}{4( {P_{2} - P_{3}} )}\frac{B_{1} - B_{2}}{B_{r}}} - \frac{B_{1}}{B_{r}}}}},} & \lbrack {{Eqn}.\quad 11} \rbrack\end{matrix}$

where P₁, P₂, and P₃ indicate the first, the second, and the thirdmeasured powers, P_(signal) indicates signal intensity in watt, S_(ASE)indicates power density of ASE in watt/nm, B₁ indicates bandwidth of ASEassociated with the first and the second power in nm, and B₂ indicatesbandwidth of ASE associated with the third power in nm, and B_(r)indicates resolution in nm, respectively.
 8. An OSNR monitoringapparatus for the optical networks comprising; 1:1 directional couplersplitting ASE-containing optical signal into two on WDM scheme-basedoptical transmission system, optical filter changing ASE bandwidth ofone of said two signals, photodetector converting, into voltage, theother of said two signals and said one signal whose bandwidth getschanged, and computer measuring OSNR using output voltage of saidphotodetector as input.
 9. An OSNR monitoring apparatus for the opticalnetworks of claim 8 further comprising; logarithmic amplifier amplifyingsaid voltage converted by said photodetector and A/D converterconverting analog quantity of said amplified voltage into digitalquantity.
 10. An OSNR monitoring apparatus for the optical networkscomprising; polarization controller controlling ASE-contained opticalsignal on WDM scheme-based optical transmission system, polarizationbeam splitter splitting said optical signal passed through saidpolarization controller into two perpendicularly polarized components,1:1 directional coupler splitting one of said two perpendicularlypolarized components into two signals, optical filter changing ASEbandwidth of one of said two signals split by said directional coupler,photodetector converting, into voltage, the other component of said twocomponents split by said polarization beam splitter, the other opticalsignal of said two signals split by said directional coupler, and saidone signal whose bandwidth gets changed by said optical filter, andcomputer measuring OSNR using output voltage of said photodetector asinput.
 11. An OSNR monitoring apparatus for the optical networks ofclaim 10 further comprising; logarithmic amplifier amplifying saidvoltage converted by said photodetector and A/D converter convertinganalog quantity of said amplified voltage into digital quantity.